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3 Human Models, Field Uniformity, and Frequency Domain

3 Human Models, Field Uniformity, and Frequency Domain

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7.2 Methods for Estimating the Induced Current Inside the Human Body



181



On the other hand, the standing human can be simulated better by an ellipsoid

rather than a spherical model: an elliptical cross section is more realistic than a circular cross section. There is also a simple formula to describe the induced current

density in elliptical cross section: J = 2πfBσ(b4 x2 + a4 y2 )1/2 (a2 + b2 )−1 , where a and

b are the semi-major and semi-minor axes on the x- and y-axes, respectively. This

formula also has been adopted by some guideline-setting bodies, such as the American Conference of Governmental Industrial Hygienists (ACGIH) and the Institute of

Electric and Electronic Engineers (IEEE): see ACGIH (1995) and IEEE (2002).

7.2.3 Numerical calculation of induced current

Recent development in computing ability has enabled large-volume numerical computation of induced currents using an anatomically accurate human body with finer

resolution. Several calculation methods for estimating the induced current inside the

human body have been developed based on Maxwell equations.

7.2.3.1 Finite element method

The finite element method (FEM) is a standard method of numerical calculation used

in many scientific fields. The advantage of this method for calculation of induced

currents inside the human body is that the FEM is suitable for simulation of the

complex shape of the human body. At Electricite de France (EdF), the TRIFOU

code has been applied to a human model with simplified internal organs (Baraton

and Hutzler 1995). The code is a combination of FEM with the boundary element

method (BEM). The FEM is applied to the human body, and the BEM is applied to

the boundary between human model and surrounding air.

7.2.3.2 Impedance method

An impedance method is a computational procedure used to solve a circuit equation

for 3-D impedance meshes that represent the human body. Gandhi and his colleagues

(Gandhi and Chen 1992; Gandhi et al. 2001) at the University of Utah and Stuchly

and her colleagues at the University of Victoria used this method in the earlier stages

of their studies (Xi and Stuchly 1994ab), applying it to an anatomical human model

having a resolution of 3.6 millimeters.

7.2.3.3 Scalar-potential, finite-difference method

A scalar-potential, finite-difference (SPFD) method is a finite-difference method

where a scalar potential is an unknown parameter. A human body is modeled by

voxels, and node equations are solved. The outer magnetic field is expressed as a

vector potential. With this approach, the amount of calculations required is relatively

small. The method is used in the studies conducted by Stuchly and her colleagues

(Dawson et al. 1997ab) and by Dimbylow (1998) at NRPB. In the NRPB study, calculation was conducted with minimum resolution of 2 mm.



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7 Induced Current as the Candidate Mechanism for Explanation of Biological Effects



7.2.3.4 Finite-difference, time-domain method

A finite-difference, time-domain (FDTD) method is used for the frequencies of the

microwave region. In the ELF region, this method is disadvantageous because of the

large number of repetitions required in the calculation. Therefore, Furse and Gandhi

(1998) introduced a frequency-scaling technique. Here calculation of induced current

is performed at 10 MHz, and the results then are converted into that of power frequency, using the linear relationship between induced current and frequency. Also,

this approach can be applied to scenarios where both magnetic and electric fields

exist simultaneously. The same method was also applied by Gustrau et al. (1999).

They used 5 MHz as the scaling frequency.

7.2.3.5 Boundary element method

A BEM (boundary element method) is used by Bottauscio and Conti (1997) at IEN

(Istituto Elettrotecnico Nazionale, Italy) and CESI (Centro Elettrotecnico Sperimentale Italiano) for a simplified human model. The advantages of a BEM are less input

data and good accuracy, provided the internal medium is simple.

7.2.3.6 Calculation method for an electrostatic problem

Accurate calculation methods that were developed originally for electrostatic problems can be applied to the problem of ELF magnetic induction in the human body, because the fundamental equation (Laplace’s equation) to be solved is the same for both

problems, provided that the quasi-static approximation is valid. That is the condition

for which displacement current can be neglected, i.e. σ ωε where ω is angular frequency and is permittivity of human model. The charge-simulation method (CSM)

and surface-charge method (SCM) are used by Yamazaki et al. (2001) of the Central

Research Institute of Electric Power Industry (CRIEPI) in Japan. These methods also

are boundary-dividing methods, and they have the advantages over volume-dividing

methods like FEM or Impedance Method in the amount of input data.

In the CRIEPI study, a simple human model constructed with axis-symmetric objects representing several major organs was used (Fig. 7.1). The effect of the organ

conductivity values assigned to each organ was investigated (Fig. 7.2), by comparing the amplitudes of the induced currents at respective organs. As expected, large

differences occur in the values of the induced current for each organ, depending on

the assumed conductivity of each organ. Example of induced field in the horizontal

crosssection of the heart is shown in Fig. 7.3.



7.3 Human Models, Field Uniformity, and Frequency Domain

In this section, unsolved problems relating to estimation of induced current inside the

human body are discussed briefly. The three main issues are (1) human model used

for induced current calculation, (2) exposure condition, and (3) frequency concerns.



7.3 Human Models, Field Uniformity, and Frequency Domain



183



Fig. 7.1. Human model used by CRIEPI. This is a simple human model constructed with

axis-symmetric objects representing five major organs (brain, heart, lung, liver, and intestine).



7.3.1 Human models

Human models used for numerical computation of induced current distribution

inside human bodies are classified into two categories. The first is an

anatomically accurate human model based on an image obtained by magnetic

resonance imaging (MRI) and a medical atlas of an anatomy. An output

(http://www.nlm.nih.gov/research/visible/visible human.html) of the US Visible

Human Project is sometimes used; it can be segmented into a resolution of 2 – 5

millimeters for computation (see Fig. 5.9). In addition, Japanese male and female

realistic models with 2 mm resolution have been developed (see Fig. 9.2, Nagaoka

et al. 2004). The second type of human model is a simplified one composed of a

relatively simple shape of an outlook and internal organ (Yamazaki et al. 2001).

The conductivity value allocated to each tissue or organ is essential for accurate

induced current calculation, because the induced current density is proportional to

the conductivity of the tissue concerned. However, the published conductivity values for each tissue or organ differ considerably, depending on the biological citation



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7 Induced Current as the Candidate Mechanism for Explanation of Biological Effects



Fig. 7.2. Example of induced field distribution on the cross sections of four human models

perpendicular to side-to-side uniform magnetic field. These models differ in assigned electric

conductivities (homogeneous and inhomogeneous models A,B,C).



selected. Moreover, in some reports, the anisotropic character of conductivity is considered for muscle.

A comprehensive investigation of tissue conductivity measurements of biological tissues was conducted by Gabriel and co-workers (Gabriel 1996, Gabriel et al.

1996abc). The output of this important work has become the standard reference for

tissue conductivity values used in modeling efforts. The use of a standard source

of tissue conductivity values reduces variability, based on choice of conductivity

values, among different models. It remains to be determined whether improved measurements providing more accuracy and increased spatial specificity can be obtained.

7.3.2 Field uniformity

In the previously mentioned studies, magnetic fields were assumed to be uniform,

allowing for easier computation and for easy comparison of the results. In addition,

in protection guidelines such as ICNIRP’s, the reference levels of magnetic exposure



7.3 Human Models, Field Uniformity, and Frequency Domain



185



Fig. 7.3. Example of induced electric field in the horizontal cross section at the center of the

heart when exposed to 1 µT, 50 Hz vertical magnetic field. The length of the arrow in the legend indicates electric field of 5 µV/m. The induced currents can be calculated by multiplying

conductivity at every position with the electric field.



are derived by assuming the magnetic field is uniform, because the coupling between

outer magnetic field and inner induced current is maximum under this condition.

On the contrary, the real-world exposures to intense magnetic fields mainly occur

in the position very close to a field source, such as (1) near a power line conductor,

in the case of a worker near a “live” line, or (2) near an electrical appliance. In

these situations, in general, the magnetic field is highly non-uniform. With a nonuniform field, the coupling between the field and the human body is relatively weak,

compared to that occurring with a uniform field.

There are several reports, in the ELF range, that take into account non-uniformity

when performing their calculations. Some reports dealt with power lines (Baraton

and Hutzler 1995; Stuchly et al. 1996; Dawson et al. 1999abc), and others describe

electrical appliances, such as a hair dryer (Baraton and Hutzler 1995; Cheng et al.

1995; Kaune et al. 1997; Tofani et al. 1995ab). Considerable work remains to be

done in the description of real-world exposure situations and in numerical modeling

of the induced currents and fields occurring in models of the human body under these

situations.



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7 Induced Current as the Candidate Mechanism for Explanation of Biological Effects



7.3.3 Expansion of frequency range studied

Resent development of appliances using magnetic field with a frequency higher than

that of the 50 or 60 Hz power system has raised a new interest in health effects. Induction heating (IH) cookers are one of the newer appliances that utilize a higher

frequency, typically 20 kHz to 100 kHz, for heating of ferromagnetic pans. Another

concern is electric article surveillance (EAS) systems installed at the entry of buildings, such as grocery stores and libraries. These devices also use these ranges of frequency. These frequency ranges are sometimes called “intermediate frequency (IF)”

mainly in the European organizations (COST 1998; Matthes et al. 1999).

Because higher frequency fields induce higher induced currents inside the human

body, with the increase proportional to the frequency, existing protection guidelines

use more strict regulation of field levels for higher frequency ranges. For example,

in the ICNIRP (1998) guideline, the reference level of magnetic exposure for the

public is 0.1 mT at 50 Hz. However, the permissible exposure is reduced greatly, to

0.00625 mT, at frequencies of a few tens of kHz. So far, only a few reports have

been published that focus on currents induced in the human body by magnetic fields

in this frequency range (Kaune et al. 1997; Gustrau et al. 1999; Gandhi and Kang

2001; Yamazaki et al. 2004).



7.4 Challenges to Interpretation of Biological Outcomes

One of the aims of estimation of the induced current occurring inside biological

object is to contribute to interpretation of the outcomes of biological experiments

with animals or cells. Stuchly and her co-workers have analyzed induced currents

in cell culture dishes considering cell membranes and gap junctions (Stuchly and

Xi 1994; Fear and Stuchly 1998ab). These efforts contribute to clarifying the microscopic dosimetry of induced current in the experimental conditions used in cell exposure. Biological investigators are coming to understand that the exposure conditions

applied to their cells are not uniform: cells at the center of a culture dish can be at

relatively low field exposure conditions while cells at the periphery of a dish, or near the

liquid interface with dish or with air, can be at relatively high field exposure conditions.

Another concern is that the biological effect caused by magnetic field can be dependent on polarization of the field (Kato et al. 1993). In this experiment, a circularly

polarized field caused the biological effect, suppression of melatonin, while linearly

polarized fields, either horizontal or vertical, did not (with the field intensities examined). Furthermore, elliptically polarized fields of various degrees of circularity

produced intermediate effects. The magnetic field near an ordinary overhead transmission line is elliptically or circularly polarized. Thus, consideration of polarity

might clarify the literature on biological effects of power-frequency magnetic fields.

There are some reports dealing with the induced currents produced by circularly

polarized currents (Misakian 1991, 1997; Yamazaki et al. 1996; Wake et al. 2000). It

should be noted that circular polarization of outer magnetic field does not necessarily

mean circular polarization of induced current inside a biological object: the polarity

of the induced current can depend on the location within the organism.



7.5 Inter-laboratory Comparison Studies



187



2



Normarized Induced Current Density (µA/m )

(Magnetic Field: 1µT, 50Hz)







+%0+42











Yamazaki et al. 2001



Gustrau et al., 1999



Baraton et al., 1995



Bottauscio et al., 1997



Dimbylow, 1998



Gandhi et al., 1992



Dawson et al., 1997a,b



Xi and Stuchly, 1994b



Xi and Stucly, 1994a







+PJQOQIGPQWU

#XGTCIG

+PJQOQIGPQWU

/CZKOWO

*QOQIGPQWU

#XGTCIG

*QOQIGPQWU

/CZKOWO



Fig. 7.4. Comparison of estimated current density inside the human body when exposed to

uniform magnetic field of 1 µT, 50 Hz. Among the nine studies examined, values varied by

almost two decades, and some exceed the ICNIRP standard.



7.5 Inter-laboratory Comparison Studies

In general, the result of numerical calculation must be verified by comparing it with

analytical or experimental values. The difficulty in the present problem is the inability to

verify results with in situ measurements. (Very little data will be available from electrodes

placed in human bodies under controlled exposure conditions.) Because of this limitation, many guideline-setting bodies have not adopted the recent, highly advanced

numerical calculation as their rationale for deriving limiting magnetic field values.

Some efforts have been conducted to compare the results of numerical calculation among different research group (Stuchly and Dawson 2000; Stuchly and Gandhi

2000; Caputa et al. 2002). The comparisons showed good agreement, provided that

similar anatomical models and conductivity values were used. However, another effort used data from nine reports and showed large variation in induced current results

among the several studies. To allow comparison, the induced current values from

each study were converted to a standard intensity of the outer uniform magnetic field

(Fig. 7.4).



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7 Induced Current as the Candidate Mechanism for Explanation of Biological Effects



7.6 Summary

For a variety of reasons, induced current has long been considered to be the most

important measure of dose when dealing with biological effects of applied or induced currents and fields. Electrostimulation has a long history in biology, dating

back to the work of Galvani and Volta in the early 19th century. Furthermore, the basic mathematical tools for describing fields and currents were developed in the 18th

century, especially by Faraday, Laplace, and Maxwell. Two centuries of biomedical

research have supported the view that current density is a prime independent variable

in bioelectromagnetics.

Given the importance of current as a basic mechanism, it has been the key component of efforts to set safety standards to protect the public against any possible

adverse health effects from environmental exposures to electric and magnetic fields.

Thus, a variety of computational models have been developed in recent years to

compute the induced current produced in the body by an external electric or magnetic field. Furthermore, the development of anatomically correct images of the human body, coupled with assignment of accurate conductivity values to all tissues

and organs, has moved the discipline from simplistic, general, back-of-the envelope

computations to what are assumed to be highly accurate and precise computational

models. The tools now available handle anatomic detail down to voxels of about 2

mm per side.

It now is relatively straight-forward to implement commercially available or

investigator-developed codes on personal computers and minicomputers. Available

approaches now include (1) finite-element methods, (2) impedance methods, (3)

scalar-potential, finite-difference methods, (4) finite-difference, time-domain methods, (5) boundary element methods, and (6) electrostatic-based computations. Using

a basic standard of 10 mA/m2 as a safety threshold for the current induced in the

human body by an external field, one can compute the induced current in all of the

various tissues or organs in the body and therefore determine if a given external field

is likely to be safe or not.

Despite dramatic progress in the last few decades, important challenges remain.

Validation is a key issue with any computational model. Direct measurement of induced current is difficult, and acquisition of additional good empirical data under a

variety of exposure conditions would be very helpful. Additionally, a 2 mm voxel

is biologically large (or even huge) for some targets; thus, finer resolution might be

very useful. Comparison of the results from various models is just beginning: sometimes agreement is good, which is encouraging. However, sometimes agreement is

not so good, which is discouraging. It is important to understand how models agree

and disagree, and it also is important to understand the causes for similarities and

differences in results from various models.

Standards would be improved by inclusion of such real-world factors as field inhomogeneity and polarity. Also, investigations need to be conducted over a wider

variety of frequencies. Because of the commercial importance of ELF power and

microwaves, these two regions have been studied more extensively than the intermediate frequencies between these extremes. As new technologies operating at different



7.7 References



189



frequencies are developed and applied, the need for safety information continues to

develop.



7.7 References

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